1. Field of the Invention
The present invention relates to a process for producing a flexible printed circuit board comprising a thin layer of a polyimide and a metallic foil, which is excellent in heat resistance, electrical characteristics and mechanical properties, and which can be free from curling, wrinkling, warping, and the like.
2. Description of the Prior Art
Prior flexible printed circuit boards have generally been produced by bonding a metallic foil such as a copper foil, etc., and an insulating film of, e.g., organic polymers, using an adhesive. However, if heat history such as thermal adhesion or the like is applied to such boards upon their production, there will result curling, twisting, warping, etc., at the time of their cooling, thus making it impossible to carry out subsequent steps such as patterning. There have also been investigated methods for producing flexible printed circuit boards without using any adhesives. It is however difficult to obtain by such methods flexible printed circuit boards which generate no curling and wrinkling resulting from temperature change.
With regard to methods for solving the problem of curling, there is disclosed, in Japanese Patent Publication (Laid Open) No. 66,966/79, a method in which a composite film consisting of polyimide and a metallic foil is subjected to heat treatment at an elevated temperature for a long period of time, and in Japanese Patent Publication (Laid Open) No. 108,272/79 there is disclosed a method wherein a composite film is wound into a cylindrical form with its resin layer outward and then subjected to heat treatment at 100.degree. to 200.degree. C. for a long period of time so as to eliminate curling. Although curling can be rectified to a considerable degree according to the above methods, putting such methods into practice can be problematic since various apparatuses are required for the aftertreatment and the addition of aftertreatment prolongs the total period of time.
There are also proposed methods in which a polyamic acid (a polyimide precursor) or a solution of a polyimide, having a linear expansion coefficient of about the same level as that of a metallic foil is directly cast on the metallic foil, followed by the removal of solvents and the curing thereof [Japanese Patent Publication (Laid Open) Nos. 206,639/85; 190,091-3/83 and 82,783/84]. However, when these methods are practiced in the air, the problem arises that the characteristics of the polyimide layer of the thus obtained printed circuit boards come to be considerably inferior to those of a polyimide film prepared separately.